Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox.
In most conventional wind turbines, the rotor hub is mounted on one end of the rotor shaft. The rotor shaft is rotatably mounted in a support structure within the nacelle on top of the wind turbine tower. The rotor thus forms an overhanging structure which transmits torque, but additionally transmits cyclical bending loads due to the loads on the blades and the weight of the hub and blades.
In the case of direct drive turbines, these bending loads can be transmitted to the generator causing air gap variations. Elimination of the gearbox in the drive train takes away an important potential source of failure. Additionally, direct drive wind turbines offer other potential advantages such as simplifying the nacelle systems, increasing reliability, and increasing efficiency.
US 20100264664 discloses a generator arrangement for a direct drive wind turbine comprising a hub with a plurality of blades, the hub being rotatably mounted relative to a support tube. The support tube may be provided with an extension mounted to the front end of the support tube. The extension may be provided with a manhole arranged between the two bearings on which the extension is mounted and via which manhole service personnel can enter the hub.
Some disadvantages of this kind of configuration relate to maintenance, i.e. handling components that need to be repaired or replaced, and to the evacuation of injured service personnel. Such operations are particularly complicated as the height of the extension of the support tube is quite low. Said working space (the extension of the support tube) may also be substantially reduced by hoisting means provided in the support tube for lowering or pulling out equipment housed inside the hub through the manhole. The available space in the support tube for service personnel may thus be relatively reduced.